The LWR camera was permanently reconfigured to the 4.5 kv configuration in October 1985. In the 4.5 kv configuration, the LWR camera is 1.37 less sensitive at all wavelengths than for the 5.0 kv configuration. This fact should be taken into account for recent LWR images. Most LWR images taken between October 1983 and 1985 were obtained with the 5.0 kv configuration, but there are exceptions.
t (LWP low disp) = 0.8·t (LWR low disp) for 2800 Å
t (LWP low disp) = 1.1·t (LWR low disp) for 2400 Å
t (LWP high disp) = 0.9·t (LWR high disp) for 2800 Å continuum
t (LWP high disp) = 0.8·t (LWR high disp) for Mg II emission lines
|1200 Å||197 DN||2100 Å||245 DN||205 DN|
Unreddened stars with spectral types B3 and earlier will produce maximum camera response at 1300Å and 2700Å in the short- and long-wavelength spectral ranges, respectively. Cooler and reddened stars will produce maximum camera response at 1800-1900 Å and 2800-3000 Å. The continuum around 1500 Å and 2200 Å will be less heavily exposed, regardless of stellar type. In the echelle (high-dispersion) mode, the intensity decreases toward the ends of each order. Hence, to obtain well-exposed spectra of a feature near the end of an order (for example, Mg II 2803Å), it may be necessary to overexpose the central part of the order. The telescope operations staff should be informed before beginning an exposure which will intentionally overexpose part of the camera.
If there is some uncertainty in the target's UV flux and a heavy overexposure (i.e. 20X or more) is needed to bring up certain spectral features, a test exposure is strongly advised. Very heavy overexposures (i.e. >500X) may permanently damage the cameras. Even at exposure levels which do not damage the cameras, phosphorescence in the UVC can affect subsequent images for up to several days after the overexposure has occurred and thus possibly affect both you and subsequent observers and their programs (see Section 4.6). This phosphorescence can be generated either from a single heavy overexposure, or a series of moderate overexposures (i.e. <2X). Project approval is required for all overexposures of >100X.
If the background radiation were averaging 1.0 volts for the last hour of the exposure, an additional 10 DN background would be added to the more sensitive regions of the camera and about 7 DN to the remaining regions resulting in a peak signal of 170 DN. If the radiation climbed above 1.0 volt, the GO might shorten the exposure since the background noise from radiation would be accumulating at a rate several times that of the target.
25 DN (Pedestal) + 7 x 7 DN/hour (Background Phosphorescence) + 7 x 13 DN/hour (Signal) = 160 DN.
t(needed) = 65 (low to high dispersion) x 2 (large to small aperture) x 0.9 (LWR to LWP high dispersion) x 30 seconds (original exposure)
t(needed) = 58.5 minutes